The present invention relates to a method for evaluating the filling ratio of a tubular rotary mill comprising a cylindrical barrel rotating around its longitudinal axis, the contents of which consist of a load of grinding medium made of metallic alloy and of material to be crushed which forms the pulp inside the mill as and when it is crushed, and essentially occupies, during the rotation of the mill and viewed in the rotation direction, the fourth trigonometric quarter of the section of the mill, while the bottom of the contents extends into the third trigonometric quarter and the top is raised into the first trigonometric quarter. The invention also relates to a device to be advantageously used for the implementation of this method.
The invention essentially aims at mills of the ball or rod mill type, in particular used for crushing clinker or for crushing coal and minerals.
To know the filling ratio of a mill is especially important for optimum operation of mining mills working in a wet process since the wear on the grinding medium is very heavy there and grinding medium has to be almost constantly supplied. This entails that the quantity of the medium still present in the mill should be known at any moment and that, consequently, a means for separately measuring the quantity of grinding medium and the quantity of pulp contained in the mill should be available.
It has been noted that optimum crushing conditions are obtained when the volume of the pulp approximately corresponds to the volume of the spaces between the pieces of grinding medium or slightly higher than this volume, without however exceeding it by more than 20%. When the volume of the pulp is too low, the crushing output is reduced and, in particular, the pieces of grinding medium that are in contact with each other mutually wear down. When the volume of pulp is too high, the crushing output is also reduced. To know the quantity of pulp in the mill therefore allows to adjust the supply of the mill in the most appropriate manner that corresponds to the optimum operation of the mill.
Among the many techniques currently known for determining the filling ratio of a mill in operation, none of them is completely satisfactory since they are generally either too imprecise or incomplete.
A first method consists in measuring the evolution in the power absorbed by the mill. This power absorbed by the mill increases with the filling ratio and reaches a maximum-after which it starts to decrease, in particular because of the reduced effect of unbalance. The power curve shows a very flat maximum, which considerably reduces the sensitivity of measurement as soon as the maximum is approached. Such a method is described in “Canadian Mineral Processors” Proceedings 1998, paper no. 24, Ottawa, Ontario.
A second method consists in measuring the forces exerted on the plating. An instrumented plate is placed within the plating and when it enters the load, the force exerted on the plate suddenly rises and decreases when the plate comes out of the load. This measurement is only applicable to mills provided with rubber plating and is very sensitive to the wear of the instrumented plate. Such a method is described in patent WO 93/00996.
Another method consists in measuring the deformation of the barrel of the mill given that it is subjected to radial and transverse deformations that increase as the mill is filled. The sensitivity of this measurement is reduced in the case of a low L/D ratio (length of the mill relative to its diameter) and by any rigidifying element such as an intermediate partition or great thickness of the barrel or of the plating. The principle of this measurement is described in the article “Measurement System of the Mill Charge in Grinding Ball Mill Circuits” by J. Kolacz-Mineral Engineering, Vol 10, No. 12, 1997 pp 1329-1338.
The installation of balances has also been considered in order to be able to take a direct measurement of the weight of the mill. However, this installation is quite difficult with existing mills.
Another method consists in measuring the noise generated by the impacts between the grinding medium and the plating of the mill. This noise increases with the filling ratio of grinding medium but, because the material to be crushed deadens the impacts, the noise decreases when the filling with material increases, hence the inaccuracy of measurement. In order to take these measurements, microphones have been used and placed near the barrel of the mill in order to detect the noises. This method is however affected by external noises (neighbouring mills in the crushing room) as well as by other factors such as the nature of the crushed material, the form of the grinding medium and the wear of the plating. Such a method is described in the article “New acoustic method for measuring the filling ratio of mill feed in tube mills” by F. Godler and J. Hagenbach, Zement-Kalk-Gyps No. 4/1994, pp E 114-E 119.
The German patent DE19933995A1 attempts to remedy the interference of the various noises by replacing the microphones with ultrasound sensors fixed to the barrel. These sensors measure the oscillations of the barrel where they are attached and not the noises transmitted through the air, which solves the problem of interfering noises.
Moreover, all the above-described methods have the drawback that they do not allow the separate evaluation of the filling ratio in grinding medium and the filling ratio of pulp or material to be crushed.
Measurement by wave absorption does in fact allow to distinguish the material to be crushed from the balls but it is not applicable to all types of material and presents a health risk because of X or gamma rays.
The aim of the present invention is to provide a new method and device allowing a reliable evaluation of the filling ratio that can easily be implemented on an existing mill and which can separately provide information on the grinding medium and on the pulp.
In order to achieve this objective, the present invention proposes a method of the kind described in the preamble, which is characterised in that an algorithm is established by means of a model and defines a relationship between the filling ratio of a mill on the one hand, and the angular positions of the bottom and top of the mill contents, as well as of its power absorbed on the other hand, in that the angular positions of the bottom and top of the contents are measured in the mill for which the filling ratio is to be determined, as well as its power absorbed, and in that the filling ratio of the mill is determined by means of these measurements and of the algorithm.
These measurements may be taken separately in order to determine the filling ratio of grinding medium and that of the pulp.
The angular positions of the bottom and top of the grinding load are determined by induction, whereas the angular positions of the bottom and top of the pulp are determined by conduction.
The device for implementing this method for evaluating the filling ratio of a mill comprising a barrel with inner plating is characterised in that the plating comprises at least one plate made of resin or elastomer, into which a detection system is integrated in order to detect the angular position at which the system enters the mill contents and the angular position at which the system comes out of the mill contents, in that the barrel comprises a sensor intended to generate a synchronisation signal with each turn of the mill, in that the signals generated by the detection system and the sensor are handled in an integrated processing device and sent by radio waves to a processing centre.
The detection device preferably comprises an inductive sensor for determining the angular positions of the bottom and top of the grinding load and a conductive sensor for determining the angular positions of the bottom and top of the pulp.
All the sensors are preferably duplicated and buried at different depths in the plates containing them so as to come into operation successively as and when the plates wear out.
Other features and characteristics of the invention will emerge from the detailed description of a preferred embodiment, presented below by way of illustration with reference to the attached figures in which: